The present invention relates to downhole drilling motor assemblies and, in particular, a stabilized downhole drilling motor housing.
In conventional oilfield drilling operations, a downhole drilling motor is often used to rotationally drive a toothed drill bit to bore the hole. The downhole motor is connected to a series of length of drill pipe which makes up the pipe string or drill string. The pipe string allows drilling mud to be pumped through the downhole motor to power the motor. The drilling mud then circulates around the drill bit and back up to the surface.
Typically, the pipe string and the various components of the downhole drilling motor are cylindrical and of a smaller diameter than the borehole, so as to permit drilling mud and cuttings to flow back to the surface in the annular space between the pipe string and the borehole, and to reduce drag as the pipe string and downhole drilling motor are rotated and moved up or down within the borehole.
Typically, the pipe string/downhole drilling motor combination has a low diameter to length ratio: the diameter can be measured in inches and the length can be measured in hundreds of feet. The pipe string/downhole drilling motor combination is therefore relatively flexible and under the longitudinal compression experienced during drilling, the pipe string/downhole drilling motor combination will tend to flex and push against the sides of the bore hole. As a result during drilling operations, when downward force is being applied to the pipe string, the unsupported pipe string and downhole drilling motor may not be centred in the borehole, which can misalign the drill bit, as is illustrated in prior art FIG. 1. When the drill bit is misaligned, it does not drill in the desired direction, and instead of following a relatively straight path, the borehole wanders in an uncontrolled manner. Typically, in oilfield drilling the goal is to drill into the petroleum bearing formation at a specific location chosen for optimum recovery of the oil or gas. The driller""s ability to do so is reduced if the path of the borehole cannot be accurately controlled.
In conventional drilling operations, this problem of misalignment of the drill bit is mitigated by positioning a lower stabilizer immediately adjacent the drill bit (38), and an upper stabilizer (40) between the pipe string and the downhole drilling motor, as is shown in prior art FIG. 1. However, this configuration is not always effective in keeping the drill bit properly aligned as the upper stabilizer is usually not sufficiently proximate to the lower stabilizer to keep the downhole drilling motor assembly centred in the borehole because the downhole drilling motor is usually 2.5, or more, meters in length and it is subject to the flexing forces imparted by the unsupported pipe string. As well, adding the upper stabilizer introduces an additional joint between components of the downhole drilling motor. These so called xe2x80x9ctool jointsxe2x80x9d are the weakest part of the downhole drilling motor assembly and it is recognized that as a general practice, the fewer tool joints the better.
Therefore, there is a need in the art for an improved downhole drilling motor system which mitigates the difficulties of the prior art.
In one aspect of the invention and in general terms, the invention is a downhole drilling motor having a power section comprising a housing and an internal motor mechanism which powers a rotating drill bit which drills a borehole, wherein said housing comprises:
(a) an internal bore enclosing the internal motor mechanism, and
(b) an external surface having at least one stabilizing rib;
wherein said stabilizing rib contacts the borehole wall when the downhole drilling motor is in use and is effective to stabilize the motor within the borehole.
The stabilizing rib or ribs may be oriented helically about the longitudinal axis of the housing. Alternatively, the stabilizing ribs may be oriented substantially parallel to the longitudinal axis of the housing in which case there may be three or more stabilizing ribs.
In another aspect of the invention, the invention comprises a downhole drilling motor having an internal motor mechanism which powers a rotating drill bit for use in drilling a borehole, and having a power section comprising a housing wherein the diameter of the smallest circle which circumscribes a cross-sectional profile of said housing along a plane normal to a longitudinal axis of said housing at any point along the length of the housing is substantially the same as the diameter of the borehole.
The housing may be configured such that the smallest circumscribing circle touches the cross-sectional profiles of said housing at at least three points. The cross-sectional profiles of the housing may be substantially a triangle or an equilateral triangle.
The housing may be configured such that the smallest circumscribing circle touches the cross-sectional profiles of said housing at four points. The cross-sectional profiles of the housing may be substantially quadrilateral or square.
The cross-sectional profiles of the housing may be substantially circular and comprise at least three projections corresponding to the at least three points touching the smallest circumscribing circle. The projections may correspond to ribs disposed longitudinally on said housing or may correspond to ribs disposed helically on said housing.
In yet another aspect of the invention, the invention comprises an elongate downhole drilling motor housing comprising an external stabilizing configuration having a stabilizing surface which contacts the walls of a circular borehole to effectively stabilize the housing within the borehole and defining passages through which drilling mud may pass upwards between the housing and the borehole walls or through the housing itself.
The stabilizing configuration may comprise at least one rib, and preferably three ribs which are disposed helically about the housing.
The stabilizing configuration may comprise at least three ribs which are disposed substantially longitudinally along the housing.
The ribs may be integral with the housing or rigidly affixed to the housing.
The ribs may be formed by the corner portions of a triangular or quadilateral cross-sectional shape of the housing.